always enable C++11

[toge]

I met compilation error octomap/1.10.0 on gcc 5.4.

compilation error

In file included from ./octomap/include/octomap/OcTreeBaseImpl.h:83:0,
                 from ./octomap/include/octomap/OccupancyOcTreeBase.h:44,
                 from ./octomap/include/octomap/OcTreeStamped.h:39,
                 from ./octomap/src/OcTreeStamped.cpp:34:
./octomap/include/octomap/OcTreeIterator.hxx:45:13: error: expected nested-name-specifier before ‘iterator_category’
       using iterator_category = std::forward_iterator_tag;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:46:13: error: expected nested-name-specifier before ‘value_type’
       using value_type = NodeType;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:47:13: error: expected nested-name-specifier before ‘difference_type’
       using difference_type = NodeType;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:48:13: error: expected nested-name-specifier before ‘pointer’
       using pointer = NodeType*;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:49:13: error: expected nested-name-specifier before ‘reference’
       using reference = NodeType&;
             ^
In file included from ./octomap/include/octomap/OcTreeBaseImpl.h:83:0,
                 from ./octomap/include/octomap/OccupancyOcTreeBase.h:44,
                 from ./octomap/include/octomap/OcTree.h:38,
                 from ./octomap/src/OcTree.cpp:34:
./octomap/include/octomap/OcTreeIterator.hxx:45:13: error: expected nested-name-specifier before ‘iterator_category’
       using iterator_category = std::forward_iterator_tag;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:46:13: error: expected nested-name-specifier before ‘value_type’
       using value_type = NodeType;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:47:13: error: expected nested-name-specifier before ‘difference_type’
       using difference_type = NodeType;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:48:13: error: expected nested-name-specifier before ‘pointer’
       using pointer = NodeType*;
             ^
./octomap/include/octomap/OcTreeIterator.hxx:49:13: error: expected nested-name-specifier before ‘reference’
       using reference = NodeType&;
             ^

It seems to be caused by using type alias.
type alias has been introduced C++11. ref

This PR ensures that C++11 is always used on CMakeLists.txt.

[toge]

Thanks!